1. Field of the Invention
The present invention relates to a ball nose end mill, and more specifically, to an indexable type ball nose end mill used for high speed, high precision machining of ball groove of a uniform joint and the like.
2. Description of the Background Art
Conventionally, when high speed and high precision machining is required in cutting a ball groove, for example, a solid ball nose end mill in which a holder body and a cutting edge are integrated, or a tipped ball nose end mill in which a cutting edge is fixed on a holder body by brazing, for example, has been used. In such ball nose end mills, however, when the cutting speed is increased to improve cutting efficiency, wear or chipping of the cutting edge increases. Therefore, in the ball nose end mills described above, as the degree or amount of wear or chipping increases, it becomes necessary to regrind the cutting edge or to exchange the ball nose end mill body more frequently, resulting in low economical efficiency.
In view of the foregoing, an indexable type ball nose end mill has come to be widely used, in which an indexable insert having a cutting edge formed on an indexable plate-shaped insert is mounted. The indexable type ball nose end mill is used with the plate-shaped insert inserted into a slot at a tip end of the holder body. The plate-shaped insert is attached to the holder body by closing the slot by clamping force of one damp screw. More specifically, an end surface of the plate-shaped insert is brought into contact with a bottom surface of the slot, and the end surface of the plate-shaped insert is pressed to the bottom surface of the slot of the holder body, so that rotation of the plate-shaped insert is restricted, and the plate-shaped insert and the holder body are integrated.
More specifically, in the above described indexable type ball nose end mill, a central portion of the plate-shaped insert is fixed and pressed to the bottom surface of the slot using a clamp screw, so that the bottom surface of the slot and an end surface of the plate-shaped insert are brought into contact with each other, thereby securing the plate-shaped insert. Therefore, in order to maintain exact securing of the plate-shaped insert even when cutting force variation occurs periodically during cutting, it is necessary to improve the precision of screw-fixing by the clamp screw. With the improved clamping precision, a clamping force sufficient to maintain the friction force between the male screw thread on the clamp screw and the female screw thread on the holder body is ensured.
For this purpose, various proposals related to the shapes of portions around the clamp screw have been made. In Japanese Patent Publication No. 64-1248, a slit is provided at the center of the bottom surface of the slot. FIG. 14 is a cross section of a ball nose end mill in accordance with this one example of the prior art in which a slot 101 of a holder body 100 is viewed from one side. The ball nose end mill having a slit 103 at the central portion of the bottom surface 102 of slot 101 has such a structure as described below, with reference to FIG. 14.
At the center of bottom surface 102 of slot 101 at the tip end of holder body 100, there is provided a slit 103. Plate-shaped insert 104 is inserted into slot 101. The space of slot 101 is made narrow by applying the clamping force of clamp screw 105, and a rear end surface of plate-shaped insert 104 is in contact with bottom surface 102 of slot 101. Therefore, in the conventional ball nose end mill having a slit a 103 provided at the central portion of the bottom surface 102 of slot 101, plate-shaped insert 104 is fixed in tight contact with bottom surface 102 of slot 101 of holder body 100 with its mechanical rotation suppressed.
In this prior art example, as slit 103 is provided at an equal distance from opposing side surfaces of slot 101, one half 106 and the other half 107 bend equally. Accordingly, plate-shaped insert 104 is fastened firmly, as it contacts the entire area of the side surfaces of the slot 101, that is, whole areas of the inner side surface of one half 106 and the side surface of the other half 107.
This prior art, however, has the following problem. Here, half 107 is inclined with respect to clamp screw 105 because of slit 103, and therefore accurate screw-fixing between the male screw portion of clamp screw 105 and female screw portion of half 107 is not attained. Further, by the method of clamping in which only the damp screw 105 is simply passed through a through hole, there remains a space between damp screw 105 and the through hole of plate-shaped insert 104, and therefore the plate-shaped insert 104 cannot be forced onto the bottom surface 102 of slot 101. As a result, restricting force is insufficient in the direction of extension of slot 101 and in the direction of rotation of damp screw 105, and therefore plate-shaped insert 104 may possibly be displaced when cutting force varies significantly.
Another ball nose end mill having a plate-shaped insert fixed therein is disclosed in Japanese Patent Laying-Open No. 8-252714. Two types of ball nose end mills disclosed in this laid-open application will be described with reference to FIGS. 15 and 16. FIG. 15 shows an example of the ball nose end mill having the plate-shaped insert with the rear end thereof formed in an arcuate shape, and FIG. 16 shows an example of a ball nose end mill having the plate-shaped insert with the rear end thereof formed in a V-shape.
First, the technical idea for fixing the plate-shaped insert of the ball nose end mill shown in FIG. 15 will be described. The plate-shaped insert 131 having a cutting edge is inserted in a slot traversing the central portion of the tip end of holder body 130 as shown in FIG. 15. Plate-shaped insert 131 is fastened as the width of the slot is narrowed by clamp screw 132.
At this time, when the rear end surface 131a of plate-shaped insert 131 is adapted to have the same radius of curvature as the bottom surface 133 of the slot, or when the radius of curvature of rear end surface 131a is made larger than that of bottom surface 133, plate-shaped insert 131 is automatically aligned with its center line moved to a position overlapping the center line of holder body 130. This significantly improves finishing accuracy of the cutting process.
In order to attain high accuracy of cutting process, however, it is necessary to improve fitting accuracy of the through hole of plate-shaped insert 131 and neck diameter of damp screw 132. In addition, the position accuracy of the screw hole of holder body 130 must also be improved. More specifically, rear end surface 131a of plate-shaped insert 131 and the arcuate contact surface of bottom surface 133 of the slot must be in contact entirely. Though it may be possible if actual parts are gauged in selective attachment, it is difficult to attain the expected accuracy when plate-shaped insert 104, holder body 130 and clamp screw 132 are assembled at random.
The technical idea for fixing the plate-shaped insert of the ball nose end mill shown in FIG. 16 will be described in the following. A plate-shaped insert 141 having a cutting edge is inserted into a slot traversing the central portion of the tip end of a holder body 140 in the similar manner as the ball nose end mill of FIG. 15, and fastened as the slot is closed by clamp screw 142 as can be seen from FIG. 16. A rear end surface 141a of a plate-shaped insert 141 is formed to have a V-shape, and a bottom surface 143 of the slot, which is formed to have a V-shape opposing that of the rear end surface 141a, abuts the rear end surface. Therefore, the center line of the plate-shaped insert 141 is automatically moved to a position overlapping the center line of holder body 140 and aligned.
If the accuracy of the V-shaped contact surfaces of plate-shaped insert 141 and bottom surface 143 varies, however, accuracy against run out of plate-shaped insert 141 is not ensured. In order to minimize the influence of run out in a random combination of plate-shaped insert 141 and holder body 140, it is effective to carry out grinding of both V-shaped contact surfaces, i.e. the rear end surface 141a of plate-shaped insert 141 and the bottom surface 143. In order to grind the bottom surface 143 of the slot, a grinding wheel of a very small diameter which can be accommodated within the slot is necessary. The grinding wheel of a small diameter, however, has a problem in its strength, and therefore highly accurate grinding is difficult.
In the cutting process using the conventional ball nose end mill described above in connection with FIGS. 14 to 16, vibration in cutting force caused by variation in the process margin of the material to be cut or by slight vibration of the cutting edge of the plate-shaped insert is unavoidable. In such a state, if plate-shaped insert 104, 131, 141, slot 101 for example, and clamp screw 105, 132, 142 are not in tight contact with each other, the prescribed friction force for holding cannot be generated between the male screw portion, i.e. the male screw thread, and the female screw portion, i.e. the female screw thread. Thus, plate-shaped insert 104, 131, 141, slot 101 for example and clamp screw 105, 132, 142 would not be clamped with appropriate preliminary pressure. As a result, the friction force for holding generated between the male screw portion and the female screw portion which is maintained by the tensile stress of clamp screw 105, 132, 142 in its axial direction may possibly be lost by the vibration caused by the variation of cutting force. As a result, the clamp screw may become loose.
In order to prevent loosening of the clamp screw, it is necessary to attain extremely highly accurate positioning in processing the through hole of the plate-shaped insert 104, 131, 141, the female screw of holder body 100, 130, 141 and the like. Improvement in processing accuracy of these portions requires very high cost, and is therefore not practical. Therefore, it is necessary to compensate for the insufficient process accuracy and position accuracy of these portions by an improvement of the clamping structure of the plate-shaped insert.